This invention relates to digital loop transmission systems and in particular to a means for automatically testing channel units at the remote terminals of such systems.
In the basic telephone system, signals are sent to and from a central office to a subscriber on a loop which includes a remote terminal. The remote terminal includes a number of channel units which provide many functions including detection of an off-hook condition, conversion between analog and digital signals, gain, equalization, and balance.
It is important for maintenance of the network that the channel units be tested when they are first plugged into the remote terminal and thereafter at periodic intervals. In the former instance (power-up testing), it is desired not only to test the functioning of the unit, but also to identify precisely the type of channel unit being installed for inventory control.
In present loop systems, channel unit testing is typically done from the central office by sending appropriate test tones through the loop during idle intervals (see, e.g., U.S. Pat. No. 4,046,964 issued to Dougherty, et al). In more recent systems, a channel test unit is included at the remote terminal, but, again, testing is controlled by a test controller at the central office (see, e.g., U.S. Pat. No. 4,270,030 issued to Brolin, et al). Such systems, while adequate, are fairly slow due to the need for control from the central office. Also, previous testing schemes could not precisely identify the channel units being plugged into the system. Further, once a test had begun, it was impractical to monitor the tip and ring conductors of the channel unit under test to determine if an off-hook condition arose. Thus, the customer could not quickly be reconnected to the network during a test.
It is, therefore, an object of the invention to provide for automatic testing of channel units from the remote terminal.